A writing head of an optical printer (hereinafter referred to as an optical writing head) is a light source for exposing a photosensitive drum and comprises a light-emitting element array. The structure of an optical printer including an optical writing head is shown in FIG. 1. An optically conductive material (photosensitive material) such as amorphous Si is provided on the surface of a cylindrical drum 2, which is rotated at printing speed. The surface of the photosensitive material is uniformly charged with an electrostatic charger 4. Then, light corresponding to a dot image being printed is projected by an optical writing head 6 onto the surface of the photosensitive material to neutralize the charge on the area to which the light is projected to form a latent image. Next, a developer 8 deposits the toner on the photosensitive material surface in accordance with the charged pattern on the photosensitive material surface. A transfer unit 10 transfers the toner on a paper sheet 14 fed from a cassette 12. The toner on the paper sheet is thermally fixed by the heat applied by a fixer 16, and the paper sheet is sent to a stacker 18. Upon completion of transfer, on the other hand, the charge on the drum is neutralized over the entire surface with an erasing lamp 20, and the remaining toner is removed by a cleaner 22.
The construction of the optical writing head 6 is shown in FIG. 2. This optical writing head comprises a light-emitting element array 24 and a rod-lens array 26 which is an erected image, unity magnification optical system, and the lens is adapted so as to focus on the photosensitive drum 2.
The inventors of the present invention have interested in a three-terminal light-emitting thyristor having a pnpn-structure as an element of the self-scanning light-emitting device, and have already filed several patent applications (see Japanese Patent Publication Nos. 1-238962, 2-14584, 2-92650, and 2-92651.) These publications have disclosed that such a self-scanning light-emitting device has a simple and compact structure for a light source of a printer, and has smaller arranging pitch of thyristors.
The inventors have further provided a self-scanning light-emitting device having such structure that a transfer portion including a transfer element array is separated from a light-emitting portion including a light-emitting element array (see Japanese Patent Publication No. 2-263668).
Referring to FIG. 3, there is shown an equivalent circuit diagram of a chip of this type of self-scanning light-emitting device of 1200 dpi/256 elements. A transfer portion of the chip comprises transfer elements T1, T2, T3, . . . , and a light-emitting portion comprises light-emitting elements L1, L2, L3, . . . , both transfer elements and light-emitting elements being composed of three-terminal light-emitting thyristors. The structure of the transfer portion includes diode D1, D2, D3, . . . , as means for electrically coupling the gate electrodes of neighboring thyristors to each other. VGK is a power supply (normally 5 volts), and is connected to all of the gate electrodes G1, G2, G3, . . . of the thyristors in the transfer portion via a load register RL, respectively. Respective gate electrodes G1, G2, G3, . . . of the thyristors in the transfer portion are correspondingly connected to the gate electrodes of the thyristors in the light-emitting portion. A start pulse øS is applied to the gate electrode of the thyristor T1 in the transfer portion, transfer clock pulses ø1 and ø2 are alternately applied to all of the anode electrodes of the thyristors in the transfer portion, and a write signal øI is applied to all of the anode electrodes of the thyristors in the light-emitting portion.
In the figure, reference numerals 30, 32, 34, and 36 indicate ø1 line, ø2 line, øI line, and power supply line, respectively. R1, R2 and RI designate current limiting resistors inserted in ø1 line 30, ø2 line 32, and øI line 34, respectively. RS indicates a current limiting resistor for the start pulse.
The operation of this self-scanning light-emitting device will now be described briefly. Assume that as the transfer clock ø2 is driven to a high level, the thyristor T2 is now turned on. At this time, the voltage of the gate electrode G2 is dropped to a level near zero volt from 5 volts. The effect of this voltage drop is transferred to the gate electrode G3 via the diode D2 to cause the voltage of the gate electrode G3 to set about 1 volt which is the diffusion potential of the diode D2. On the other hand, the diode D1 is reverse-biased so that the potential is not conducted to the gate electrode G1, then the potential of the gate electrode G1 remaining at 5 volts. The turn on voltage of the light-emitting thyristor of pnpn-structure is approximated to a gate electrode potential + a diffusion potential of pn junction (about 1 volt). Therefore, if a high level of a next transfer clock pulse ø1 is set to the voltage larger than about 2 volts (which is required to turn-on the thyristor T3) and smaller than about 4 volts (which is required to turn on the thyristor T5), then only the thyristor T3 is turned on and other thyristors remain off-state, respectively. In this manner, on-state of transfer elements are sequentially transferred by means of two-phase clock pulses ø1 and ø2.
The start pulse øS works for starting the transfer operation described above. When the start pulse øS is driven to a low level (about 0 volt) and the transfer clock pulse ø1 is driven to a high level (about 2-4 volts) at the same time, the thyristor T1 is turned on. Just after that, the start pulse øS is returned to a high level.
Assuming that the thyristor T2 is in on-state, the voltage of the gate electrode G2 is lowered to almost zero volt. Consequently, if the voltage of the write signal øI is higher than the diffusion potential (about 1 volt) of the pn junction between gate and anode, the thyristor L2 may be turned into on-state (a light-emitting state).
On the other hand, the voltage of the gate electrode G1 is about 5 volts, and the voltage of the gate electrode G3 is about 1 volt. Consequently, the write voltage of the thyristor L1 is about 6 volts, and the write voltage of the thyristor L3 is about 2 volts. It follows from this that the voltage of the write signal øI which can write into only the thyristor L2 is in a range of about 1-2 volts. When the thyristor L2 is turned on, that is, in the light-emitting state, the amount of light thereof is determined by the current value supplied by the write signal øI. Accordingly, the thyristors may emit light at any desired amount of light. In order to transfer on-state to the next thyristor in the light-emitting portion, it is necessary to first turn off the thyristor in on-state by temporarily dropping the voltage of the write signal øI down to zero volt.
A self-scanning light-emitting element array in an optical writing head may be fabricated by arranging a plurality of chips described above in a linear manner. As apparent from the operation described above, the number of light-emitting elements which may be illuminated simultaneously in one chip is only 1.
In order to make the printing speed of an optical printer fast, it is required to increase an energy exposed on a photosensitive drum. An exposure energy is a product of an optical output (which has a dimension of power) and an exposure time, so that it is required to increase an optical output or an exposure time in order to make an exposure energy large. A current applied to a light-emitting element is caused to be increased to make an optical output large, but it is not permitted to increase extremely the current due to the effect for a lifetime of the light-emitting element. On the other hand, the number of light-emitting elements which may be illuminated simultaneously in one chip is required to be increased in order to extend an exposure time, i.e. increase a light emission duty.
An object of the present invention is to provide a method for driving a self-scanning light-emitting element array in such a manner that two or more light-emitting elements may be illuminated simultaneously in one chip.
Another object of the present invention is to provide a self-scanning light-emitting array in which two or more light-emitting elements may be illuminated simultaneously in one chip.
A diode-coupled self-scanning light-emitting element array shown in FIG. 3 is structured so as to be driven by a driver IC (Integrated Circuit) of a 5V power supply system. The voltage of a power supply for a driver IC, however, has been changed from a 5V power supply system to a 3.3V or lower power supply system, which has decreased a power consumption. It is, therefore, desirable to drive a self-scanning light-emitting element array by a 3.3V power supply system.
A still another object of the present invention is to provide a method for driving a diode-coupled self-scanning light-emitting element array by a 3.3V power supply system, and a driver circuit for implementing the method.
An optical writing head is composed of a light-emitting element array and a rod-lens array. When the light-emitting element array is structured by arraying a plurality of self-scanning element array chips in a linear manner with the ends of adjacent chips being butted to each other to form junctions, it is impossible to make the array pitch of light-emitting elements constant over the light-emitting element array, especially an array pitch is disordered at the junctions. In order to avoid this, the chips are arrayed in a zigzag manner with the ends of each chip being overlapped to one another to make an array pitch of light-emitting elements at the junctions of chips constant.
When a printing is carried out by an optical writing head comprising such a light-emitting element array, stripes may be printed on a paper sheet at the junctions of chips.
A further object of the present invention is to provide an optical writing head in which the stripes due to above-described reason are not printed on a paper sheet.
A still another object of the present invention is to provide a method for arranging a rod-lens array and a light-emitting element array to implement the above-described optical writing head.